511 Compute inflow and outflow of models

[HenrZu]

• implementation of flows between the different infection states
• remove totalinfection compartment (can be computed afterwards using the flows)

Merge Request - GuideLine Checklist

Guideline to check code before resolve WIP and approval, respectively.
As many checkboxes as possible should be ticked.

Checks by code author:

Always to be checked:

• There is at least one issue associated with the pull request.
• The branch follows the naming conventions as defined in the git workflow.
• New code adheres with the coding guidelines
• Make sure that the pre-commit linting/style checks pass.

If functions were changed or functionality was added:

• Tests for new functionality has been added
• A local test was succesful

If new functionality was added:

• There is appropriate documentation of your work. (use doxygen style comments)

If new third party software is used:

• Did you pay attention to its license? Please remember to add it to the wiki after successful merging.

If new mathematical methods or epidemiological terms are used:

• Are new methods referenced? Did you provide further documentation? Has the glossary been updated?

The following questions are addressed in the documentation if need be:

• Developers (what did you do?, how can it be maintained?)

• For users (how to use your work?)

• For admins (how to install and configure your work?)

• For documentation: Please write or update the Readme in the current working directory!

Checks by code reviewer(s):

• Is the code clean of development artifacts e.g., unnecessary comments, prints, ...
• The ticket goals for each associated issue are reached or problems are clearly addressed (i.e., a new issue was introduced).
• There are appropriate unit tests and they pass.
• The git history is clean and linearized for the merge request. All reviewers should squash commits and write a simple and meaningful commit message.
• Coverage report for new code is acceptable.

closes #511
closes #317

[HenrZu]

Thank you @dabele and @mknaranja for reviewing 🥳